Data separator having a restart circuit

ABSTRACT

A data separator for use in extracting data from a read signal includes a restart circuit. The data separator is comprised of a controlled oscillator, a delay circuit, a restart circuit, a phase detector and a data extractor. The delay circuit delays the read signal by an interval approximately equal to the time required to start the controlled oscillator after assertion of the start signal. Accordingly, the oscillator starts in-phase with the delayed read signal. In one embodiment of the present invention, the delay interval of the delay circuit is controlled by a frequency synthesizer which generates a write clock during write operations.

BACKGROUND OF THE INVENTION

The present invention relates to a data separator for use in reading data from a disk drive. More particularly, the present invention relates to a restart circuit which starts a voltage controlled oscillator in-phase with a signal from a disk drive read head.

In the art of data recording, data is stored in a disk drive system by positioning a read/write transducer head proximate a rotating magnetic disk. A write signal is applied to the read/write head causing the head to emit a transient magnetic field, which creates a series of magnetic polarizations on the disk surface. The write signal is created by synchronizing the data to be written with a write clock signal. Typically, the write clock signal itself is not written to the disk surface.

When reading data from the disk, the read/write head passes over the magnetic polarizations. This induces a transducer signal in the read/write head. The transducer signal is processed to form a read signal. Because the write clock signal which originally synchronized the data is not itself written to the disk, a read clock signal must be recreated to frame the data embedded in the read signal. The read clock signal must compensate for variations in the rate at which data is read from the disk. These variations are typically caused by imperfections in the servo mechanism that rotates the disk, and by vibrations.

It is known in the art to utilize a phase locked loop to create a read clock signal which can frame the data embedded in the read signal. A typical phase locked loop is comprised of a voltage controlled oscillator (VCO), a phase detector, a charge pump and a loop filter. The VCO receives an input voltage and produces an output signal having a frequency which varies with the magnitude of the input voltage. A read clock signal is derived from the VCO output signal (typically by applying the VCO output signal to a frequency divider). The phase detector compares the phase of the read clock signal with the phase of the read signal.

If the phase of the read clock signal lags the phase of the read signal, the phase detector will issue a pump-up signal to the charge pump. The pump-up signal raises the voltage delivered to the VCO input, thereby raising the frequency of the VCO output signal and the read clock signal. If the phase of the VCO output signal leads the phase of the read signal, then the phase detector will issue a pump-down signal to the charge pump. The pump-down signal lowers the voltage supplied to the VCO, thereby lowering the frequency of the VCO output signal and the read clock signal. Accordingly, the read clock signal will eventually lock on to the read signal.

When the read/write head begins reading from the disk surface, it is desirable to have the VCO lock on to the phase and frequency of the external input signal as quickly as possible Minuhin et al, U.S. Pat. No. 4,875,108, discloses a phase locked loop wherein the initial phase error between a VCO signal and an external input signal will be no more than one-eighth of a detection window.

The phase locked loop disclosed by Minuhin employs a VCO which provides four output signals, each at the same frequency, but phase shifted by 90° from each other. When the external input signal transitions from one voltage level to another, a selection apparatus selects the VCO output having the closest phase to the external input signal. As a result, the initial phase error will be no more than one-eighth of a detection window.

Another technique known in the art attempts to minimize the initial phase error between an external input signal and a VCO output signal by stopping the VCO and thereafter starting the VCO based on a transition from one voltage level to another voltage level in the external input signal. By starting the VCO based on a transition in the external input signal, the phase difference between the VCO output signal and the external input signal is reduced.

One problem associated with stopping and starting the VCO is that a typical VCO will not start oscillating immediately after receiving a start signal. A typical VCO is comprised of a pair of transistors coupled in a regenerative feedback loop such that a capacitor is continuously charged and discharged. This type of VCO is typically stopped by using a transistor to break the regenerative feedback loop, thereby causing circuit nodes in the VCO to become clamped at certain voltages.

Unfortunately, the simplest and most stable methods of clamping the voltages introduce a large VCO startup interval. Attempts have been made to minimize the startup interval by clamping circuit nodes in the VCO at voltages which are closer to the voltages at which the regenerative feedback loop would switch states. However, these methods require additional circuitry and tend to stop the VCO at a point which is inherently less stable. In addition, such methods can never completely eliminate an initial phase error. It is nearly impossible to clamp voltages at circuit nodes in the VCO at levels which would cause the VCO output signal to instantly transition from one state to another upon receiving the start signal.

SUMMARY OF THE INVENTION

The present invention is a data separator which includes a restart circuit. Based on a read request signal, the data separator extracts data embedded in a read signal which is derived from a transducer signal that is generated by a transducer moving proximate a storage media. The data separator is comprised of a controlled oscillator, a delay circuit, a restart circuit, a phase detector and a data extractor.

The controlled oscillator produces an oscillator output signal which has an oscillation frequency that varies in response to a frequency control signal. The controlled oscillator is also responsive to a stop signal which stops oscillation and a start signal which starts oscillation. A start interval exists between assertion of the start signal and oscillation of the oscillator. A read clock signal, which is used by the data extractor to extract data from the read signal, is derived from the oscillator output signal.

The delay circuit delays the read signal by a delay interval, producing a delayed read signal. The delay interval is approximately equal to the start interval of the controlled oscillator. The phase detector controls a phase difference between the delayed read signal and the read clock signal by modulating the frequency control signal. The data extractor extracts data from the read signal by framing data in the read signal using the read clock signal.

The restart circuit is responsive to the read request signal and the read signal. Upon detecting the read request signal, the restart circuit will assert the stop signal of the controlled oscillator and thereafter assert the start signal of the controlled oscillator based on a transition from one signal level to another signal level in the read signal. Accordingly, the controlled oscillator starts oscillating in-phase with the delayed read signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram showing a data separator and a frequency synthesizer, all of which are part of a disk drive system.

FIG. 2 is a timing diagram showing various output signals of the data separator and frequency synthesizer of FIG. 1.

FIG. 3 is a simplified transistor level diagram showing an embodiment of a voltage controlled oscillator (VCO), which is part of the data separator of FIG. 1.

FIG. 4 is timing diagram showing the voltages present at certain circuit nodes of the VCO of FIG. 3.

FIG. 5 is a timing diagram showing the voltages present at certain circuit nodes of the VCO of FIG. 3 as the VCO is restarted.

FIG. 6 a simplified transistor level diagram showing an embodiment of a VCO which is part of the frequency synthesizer of FIG. 1.

FIG. 7 is a timing diagram showing the voltages present at certain circuit nodes of the VCO of FIG. 6.

FIG. 8 is a transistor level diagram showing an embodiment of a delay circuit, which is part of the data separator of FIG. 1.

FIG. 9 is a timing diagram showing the voltage present at certain circuit nodes of the delay circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified block diagram showing data separator 10 and frequency synthesizer 12, all of which are part of a disk drive system. Data separator 10 and frequency synthesizer 12 are on a common integrated circuit. The remainder of the disk drive system is not shown in FIG. 1. Data separator 10 receives a read request signal (RD₋₋ REQ) from a disk drive controller and subsequently processes a read signal (ENDAT) which is derived from a transducer signal which originates from a read/write transducer head. The ENDAT signal contains data and clock information. The RD₋₋ REQ signal is asserted by the controller when valid data is available in the ENDAT signal. Data separator 10 extracts and decodes data from the ENDAT signal and provides read data signal RD and read clock signal RC to other circuitry (not shown in FIG. 1) for additional processing.

Frequency synthesizer 12 provides write clock signal WC, which is used by other circuitry (not shown in FIG. 1) during write operations. Frequency synthesizer 12 includes control register 16, reference oscillator 18, dividers 20, 22 and 24, phase/frequency detector 26 and voltage controlled oscillator (VCO) 30. Control register 16 is coupled to the controller and stores integers A and B. A and B are each represented by five data bits and therefore can each assume a value between one and thirty-two. Integer A is supplied to divider 20 and integer B is supplied to divider 22. Divider 20 receives a reference frequency signal (F_(REF)) from reference oscillator 18 and produces a signal (F_(A)) having a frequency: ##EQU1## Divider 22 receives an output (F_(VCO)) signal from VCO 30 and produces a signal (F_(B)) having a frequency: ##EQU2##

VCO 30 produces the signal F_(VCO), which has a frequency that varies with the magnitude of the voltage supplied by phase/frequency detector 26. Phase/frequency detector 26 compares the phase of signal F_(A) with the phase of signal F_(B). If the phase of F_(A) lags the phase of F_(B), phase/frequency detector 26 lowers the voltage provided to VCO 30. If the phase of F_(A) leads the phase of F_(B), phase/frequency detector 26 raises the voltage provided to VCO 30.

The continuous comparisons performed by phase/frequency detector 26 will eventually cause the frequency of F_(B) to lock on to, and therefore equal the frequency of F_(A). Since: ##EQU3##

Accordingly, by changing A and B, the controller can accurately vary the frequency provided by frequency synthesizer 12. This capability is required for zone density recording. In zone density recording, the rate at which data is written to and read from the disk surface varies with the radial position of the read/write transducer head. In general, values for A and B are predetermined for any particular radial position. The same values for A and B are used during both read and write operations.

The primary function performed by frequency synthesizer 12 is to provide a write clock signal when writing to the disk. In contrast, data separator 10 recreates a read clock signal when reading from the disk. Data separator 10 is comprised of delay circuit 32, phase/frequency detector 34, voltage controlled oscillator (VCO) 38, zero phase restart 40, divider 42 and data extractor 46.

Delay circuit 32 provides a variable delay of the ENDAT signal to phase/frequency detector 34 and zero phase restart 40. The delay is controlled by the signal I_(REF), which is produced by VCO 30. When data separator 10 is in idle mode, phase/frequency detector 34 is operated in "phase/frequency" mode and a multiplexer in delay circuit 32 connects delay circuit 32 to the output of divider 24. In "phase/frequency" mode, phase/frequency detector 34 controls VCO 38 to cause the output of divider 42 to achieve the same phase and frequency as the output of divider 24. This allows the data separator to remain locked onto a signal having a frequency extremely close to the frequency of an impending read signal, which helps data separator 10 to quickly lock onto the read signal.

When delay circuit 32 is not in idle mode, phase/frequency detector 34 is operated in "phase only" mode. In phase only mode, phase/frequency detector 34 only performs a phase comparison when it is signaled to do so, and controls VCO 38 based on non-periodic phase comparisons.

Phase/frequency detector 34, VCO 38 and divider 42 form a phase locked loop. Phase/frequency detector 34 is signaled to perform a phase comparison when a transition from one voltage level to another voltage level in the ENDAT signal is detected entering delay circuit 32. Then phase/frequency detector 34 compares the phase of the signal delayed by delay circuit 32 with the phase of the signal from divider 42. Divider 42 divides the output signal of VCO 38 by two. VCO 38 produces a signal having a frequency which varies with the magnitude of the voltage provided by phase/frequency detector 34. Based on continuous phase comparisons, phase/frequency detector 34 will either raise of lower the voltage provided to VCO 38 to cause the output signal of divider 42 to become in-phase with the signal from delay circuit 32.

In the embodiment described herein, phase/frequency detector 34 compares a high-to-low transition in the signal from delay circuit 32 with a high-to-low transition in the signal from divider 42. In other embodiments, a low-to-high transition is compared to a high-to-low transition, a high-to-low transition is compared to a low-to-high transition and a low-to-high transition is compared to a low-to-high transition. The word "in-phase" encompasses these phase relationships, along with any other fixed phase relationship.

The output signals from divider 42 and delay circuit 32 are also provided to data extractor 46. Data extractor 46 uses the signal from divider 42 as a read clock signal to frame and extract data from the output of delay circuit 32 and provides read data signal RD and read clock signal RC to other circuitry (not shown in FIG. 1) for additional processing.

Zero phase restart 40 is provided to assist the phase locked loop in initially locking onto a read signal. The operation of zero phase restart 40 will be explained with reference to the timing diagram shown in FIG. 2.

FIG. 2 is a timing diagram showing the output signals of delay circuit 32 and divider 42 (labeled as Delay 32 and VCO 38 ÷2, respectively) and the signals RD₋₋ REQ and ENDAT. Also shown in FIG. 2 is the input signal provided to delay circuit 32.

When the disk controller prepares to read data from the disk, it positions the read head above the desired track, detects a valid signal and asserts the RD₋₋ REQ signal. The RD₋₋ REQ signal is asserted in FIG. 2 at edge 50.

Before RD₋₋ REQ is asserted, delay circuit 32 is coupled to the output of divider 24 of frequency synthesizer 12 (VCO 30÷2). As previously described, integers A and B of control register 16 determine the oscillation frequency of frequency synthesizer 16. Since integers A and B are predetermined for any particular radial position on the disk surface and the output of divider 42 tracks the output of divider 24, VCO 38 oscillates at a frequency extremely close to the frequency at which it will operate when reading data.

After RD₋₋ REQ is asserted and upon detecting the next transition from one voltage level to another voltage level in the ENDAT signal, zero phase restart 40 halts VCO 38 and switches the input of delay circuit 32 to the ENDAT signal. Although VCO 38 is stopped, phase/frequency detector 34 continues to provide the same voltage level which caused VCO 38 to oscillate at the same frequency as VCO 30, so that VCO 38 will initially oscillate at this frequency when it is restarted. This frequency will be extremely close to the frequency at which data will be read from the disk. When zero phase restart 40 detects the next transition form one voltage level to another voltage level in the ENDAT signal, it restarts VCO 38 in-phase with the signal from delay circuit 32.

The accuracy of delay circuit 32 is critical. If the delay is in error, phase/frequency detector 34 may perform a phase comparison on the wrong edge of the output of divider 42. Also, zero phase restart 40 may restart VCO 38 at the wrong time, thereby creating a phase error which the phase locked loop will require extra time to correct.

Zero phase restart 40 and phase/frequency detector 34 require different delays from delay circuit 32. Phase/frequency detector 34 requires a delay approximately equal to one period of oscillation of VCO 38, or equivalently, one-half of a period of the signal provided by divider 42. Zero phase restart 40 requires a delay smaller than the delay required by phase/frequency detector 34. With reference to FIGS. 3 and 4, it will be shown that the delay required by zero phase restart 40 is three-fourths the delay required by phase/frequency detector 34.

FIG. 3 is a simplified transistor level diagram showing an embodiment of VCO 38 of FIG. 1. In this Figure, VCO 38 is comprised of load networks 52, transistors Q₁₁, Q₁₂, and Q₁₃, capacitor C₁₁ and current supplies I₁₁ and I₁₂. I₁₁ and I₁₂ each provide a current I. FIG. 4 is timing diagram showing the voltages present at circuit nodes V_(1A), V_(1B) of V_(1C) of VCO 38.

The output of VCO 38 is derived from the voltage found at circuit node V_(1C). When the signal from zero phase restart 40 is low, V_(1C) oscillates between V_(HIGH) and V_(LOW), as shown in FIG. 4. The difference between V_(HIGH) and V_(LOW) is V_(SWING), which is determined by load networks 52.

When V_(1C) is equal to V_(HIGH), transistor Q₁₁ is on and transistor Q₁₂ is off. Accordingly, the voltage at V_(1A) is equal to V_(HIGH) minus the base-to-emitter drop of transistor Q₁₁, or:

    V.sub.1A =V.sub.HIGH -V.sub.BE.                            Eq. 7

Because transistor Q₁₂ is off, current source I₁₂ must pull current I through capacitor C₁₁. This will cause the voltage at point V_(1B) to swing from:

    V.sub.HIGH +V.sub.SWING -V.sub.BE                          Eq. 8

(the point at which transistor Q₁₁ turns on and transistor Q₁₂ turns off) to:

    V.sub.LOW -V.sub.BE                                        Eq. 9

(the point at which transistor Q₁₁ turns off and transistor Q₁₂ turns on). When transistor Q₁₁ turns off and transistor Q₁₂ turns on, V_(1C) switches from V_(HIGH) to V_(LOW), and the voltage swing just observed at node V_(1B) is repeated at node V_(1A). This cycle will continuously repeat, thereby providing a stable oscillator output at point V_(1C).

FIG. 5 is a timing diagram showing the voltages at nodes V_(1A), V_(1B), and V_(1C) as VCO 38 is restarted by releasing the signal from zero phase restart 40. In FIG. 5, the zero phase restart signal is initially high. When this signal is high, transistors Q₁₃ and Q₁₂ of FIG. 3 are on and transistor Q₁₁ is off. The regenerative feedback path between transistors Q₁₁ and Q₁₂ is broken and the voltages at nodes V_(1A) and V_(1B) are clamped. Zero phase restart 40 must provide a voltage of at least V_(HIGH) to stop VCO 38. The actual voltage provided by zero phase restart 40 is:

    V.sub.HIGH +V.sub.NOISE,                                   Eq. 10

where V_(NOISE) is a voltage sufficient to mask any noise in the circuit and prevent transistor Q₁₁ from turning on.

When the signal from zero phase restart 40 is high, the voltage at node V_(1C) is V_(LOW). The voltage at node V_(1A) is equal to the voltage of the zero phase restart signal minus the base-to-emitter drop of transistor Q₁₃, or equivalently:

    V.sub.A1 =V.sub.HIGH +V.sub.NOISE -V.sub.BE. Eq. 11

The voltage at node V_(1B) is:

    V.sub.B1 =V.sub.HIGH -V.sub.BE,                            Eq. 12

which is the same voltage that is present at V_(1B) when VCO 38 is oscillating and transistor Q₁₂ is on.

As the signal from zero phase restart 40 goes low, transistor Q₁₃ turns off. Transistor Q₁₂ remains on and transistor Q₁₁ remains off. Accordingly, the voltage at node V_(1C) remains at V_(LOW) and the voltage at node V_(1B) remains at:

    V.sub.HIGH -V.sub.BE.                                      Eq. 13

However, the current provided by current source I₁₁ must now pass through capacitor C₁₁ because transistor Q₁₃ is off. The voltage at node V_(1A) begins dropping from:

    V.sub.HIGH +V.sub.NOISE -V.sub.Be                          Eq. 14

until it reaches:

    V.sub.LOW -V.sub.BE.                                       Eq. 15

At this point, transistor Q₁₁ turns on, transistor Q₁₂ turns off, the voltage at node V_(1C) switches from V_(LOW) to V_(HIGH) and VCO 38 resumes oscillation.

In FIG. 5, the time interval between the high-to-low transition of the zero phase restart signal and the low-to-high transition in the voltage at node V_(1C) is shown as ΔT. ΔT is equal to the time it takes for the voltage at node V_(1A) to drop from;

    V.sub.HIGH +V.sub.NOISE -V.sub.BE                          Eq. 16

    to:

    V.sub.LOW -V.sub.BE.                                       Eq. 17

To derive an approximate value for ΔT, it can be assumed that V_(NOISE) is zero. As seen in FIGS. 4 and 5, the length of time required for the voltage at node V_(1A) to drop from:

    V.sub.HIGH -V.sub.BE                                       Eq. 18

    to:

    V.sub.LOW -V.sub.BE                                        Eq. 19

is equal to one-fourth of a VCO 38 oscillation period. Therefore, ΔT is approximately equal to one-fourth of a VCO 38 oscillation period, and more accurately, one-fourth of a period plus the time required for the voltage at node V_(1A) to drop by V_(NOISE).

Accordingly, zero phase restart 40 of FIG. 1 must provide a restart signal to VCO 38 approximately one-fourth of a VCO 38 oscillation period before a transition in the signal provided to phase/frequency detector 34 by delay circuit 32. Therefore, delay circuit 32 must delay the signal sent to zero phase restart 40 by approximately three-fourths of a VCO 38 oscillation period.

As described above, delay circuit 32 provides different delays to phase/frequency detector 34 and zero phase restart 40. While these delays are different, they are related by the oscillation period of VCO 38. These delays must also be variable, in accordance with the requirements of zone density recording. To vary the delays, integers A and B of dividers 20 and 22 are set to the predetermined values required for the radial position at which data is being read from the disk. Accordingly, VCO 30 will oscillate at the same frequency as it did when the data was written to the disk. A reference current I_(REF) is provided to delay circuit 38 from VCO 30. Based on the reference current I_(REF), delay circuit 32 provides delays which are related to the oscillation period of VCO 30, which is the same as the expected oscillation period of VCO 38.

FIG. 6 a simplified transistor level diagram showing an embodiment of VCO 30 of FIG. 1. VCO 30 is comprised of resistor R₁, diodes D₁, D₂, D₃ and D₄, transistors Q₁, Q₂, Q₃, and Q₄, current sources I₁, I₂, I₃, I₄, I₅ and I₆, capacitor C₁ and transconductance amplifier T.

Transconductance amplifier T modulates current sources I₁, I₂, I₃, I₅ and I₆ based on a voltage V_(IN) received from phase/frequency detector 26 of FIG. 1. The currents produced by current sources I₁, I₂, I₅ and I₆ are equal, with each current defined by:

    I=K·(V.sub.IN -V.sub.1),                          Eq. 20

where V₁ is an internal reference voltage and K is a transconductance constant. Current source I₃ is modulated to produce a current 2.I.

Transistors Q₁ and Q₂ form a regenerative feedback circuit such that only one of the transistors is on at a time. Transistors Q₃ and Q₄ mirror the states of transistors Q₁ and Q₂, respectively, so that when transistor Q₁ is on transistor Q₃ is on, and when transistor Q₂ is off transistor Q₄ is off. Conversely, when transistor Q₁ is off transistor Q₃ is off, and when transistor Q₂ is on transistor Q₄ is on. For the purpose of describing the circuit, it will be assumed that transistors Q₁ and Q₃ are initially on and transistors Q₂ and Q₄ are initially off.

With transistor Q₂ off, current I supplied by current source I₆ must pass through diode D₂ and current source I₃. Since current source I₃ is modulated to produce a current of 2.I, a current I also passes through diodes D₃ and D₄. The voltage at circuit node V_(E) is V_(CC) minus the voltage drops over diodes D₃ and D₄. In the technology used in the preferred embodiment of the present invention, a voltage drop over a diode is equal to the base-to-emitter drop of a transistor. Therefore, the voltage at node V_(E) is:

    V.sub.E =V.sub.CC -2V.sub.BE.                              Eq. 21

Since transistor Q₂ is off, the voltage at node V_(D) is the voltage at node V_(E) plus the voltage drop over diode D₂. The voltage at node V_(D) is given by:

    V.sub.D =V.sub.CC -2V.sub.BE +V.sub.BE                     Eq. 22

    or

    V.sub.D =V.sub.CC -V.sub.BE.                               Eq. 23

A reference voltage created at node V_(F) is applied to the bases of transistors Q₃ and Q₄. This reference voltage is created by current source I₄ pulling a current I_(R) over resistor R₁. A voltage drop V_(R) is formed over resistor R₁ such that the voltage at node V_(F) is:

    V.sub.F =V.sub.CC -V.sub.R.                                Eq. 24

Current source I₄ and resistor R₁ are designed such that the voltage drop V_(R) over resistor R₁ does not vary with temperature or power supply variations. Such designs are well known in the art. The resistance of resistor R₁ and current I_(R) of current source I₄ are chosen such that:

    V.sub.R <V.sub.BE.                                         Eq. 25

Accordingly, transistor Q₄ remains off.

With transistor Q₂ off, transistor Q₁ conducts a current of 2.I; a current I through current source I₁ and a current I through current source I₂ via capacitor C1. The voltage of node V_(B) with respect to node V_(A) will decrease by: ##EQU4##

Since a current of 2.I is flowing through transistor Q₁ and a current of I is flowing through current source I₅, a current I must also be flowing through transistor Q₃. Therefore, transistor Q₃ is on and the voltage at node V_(C) is given by:

    V.sub.C =V.sub.CC -V.sub.R -V.sub.BE.                      Eq. 27

Since the voltage at node V_(C) is higher than the voltage at node V_(E), diode D₁ is reverse-biased and is not conducting current.

The voltage at node V_(A) is:

    V.sub.A =V.sub.D -V.sub.BE                                 Eq. 28

    or

    V.sub.A =V.sub.CC -2·V.sub.BE.                    Eq. 29

The voltage at node V_(B) will continue to ramp down until transistor Q₂ begins to conduct. Transistor Q₂ will begin conducting when:

    V.sub.B =V.sub.C -V.sub.BE                                 Eq. 30

    or

    V.sub.B =V.sub.CC -V.sub.R -2·V.sub.BE.           Eq. 31

As transistor Q₂ begins to conduct, the voltage drop over current source I₆ begins to increase, causing the voltage at node V_(D) to drop. When the voltage at node V_(D) drops by V_(BE), transistor Q₁ shuts off and transistor Q₄ turns on. When transistor Q₁ shuts off, the voltage drop over current source I₅ decreases. This decrease causes the voltage at node V_(C) to increase, which reinforces the "on" state of transistor Q₂ and turns off transistor Q₃. Diode D₁ begins to conduct and diode D₂ becomes reverse biased. The current I which previously passed through diode D₂ now passes through diode D₁.

At this point, the original assumptions are reversed transistors Q₂ and Q₄ are now on and transistors Q₁ and Q₃ are now off. Just before the transistors switched states, the voltages at nodes V_(A) and V_(B) were:

    V.sub.A =V.sub.CC <2·V.sub.BE                     Eq. 33

    and

    V.sub.B =V.sub.CC -V.sub.R -2·V.sub.BE.           Eq. 33

Therefore,

    V.sub.A -V.sub.B =V.sub.R.                                 Eq. 34

Just after the transistors switched, the voltage at node V_(B) assumed the same voltage level previously found at node V_(A), and the voltage over capacitor C1 was capacitively coupled to node V_(A). Therefore the voltage at node V_(A) is given by:

    V.sub.A =V.sub.CC -2·V.sub.BE +V.sub.R.           Eq. 35

The voltage at V_(A) will now decrease until:

    V.sub.A =V.sub.CC -2·V.sub.BE -V.sub.R,           Eq. 36

with the voltage at node V_(A) dropping by 2·V_(R). At this point, the transistors will switch states, a complete cycle will have been completed and VCO 30 returns to the state set forth in the original assumption.

The waveforms present at nodes V_(A), V_(B), V_(C) and V_(D) while VCO 30 oscillates are shown in FIG. 7. As shown in FIG. 7, one-half of a period T_(V) occurs when the voltage at node V_(A) (or equivalently node V_(B)) drops by 2·V_(R). Therefore, ##EQU5##

As described above, the rate at which the voltage drops is: ##EQU6##

Accordingly, the period of VCO 30 is directly proportional to the reference voltage V_(R) and the capacitance C of capacitor C1, and inversely proportional to the current I. Through the action of the phase locked loop of frequency synthesizer 12 of FIG. 1, current I will be adjusted to cause VCO 30 to oscillate at a frequency dictated by oscillator 18 and dividers 20 and 22. As described above, the reference voltage V_(R) is designed to be a constant. The capacitance C of capacitor C1 is typically subject to process variations of approximately 20 percent. However within any integrated circuit, the process variations between capacitors are much smaller, typically 0.1 to 0.5 percent. VCO 30 and delay cell 32 of FIG. 1 are on a common integrated circuit. Therefore, the current I can be used to accurately control delay circuit 32 because process variations in delay circuit 32 will track process variations in VCO 30.

FIG. 8 is a transistor level diagram showing an embodiment of delay circuit 32 of FIG. 1. This embodiment receives the I_(REF) signal produced by the embodiment of VCO 30 shown in FIG. 6. Delay circuit 32 is comprised of multiplexer M₁, NOR gates G₁, G₂ and G₃, transistor Q₅, capacitor C₂, current sources I₇, I₈ and I₉, diodes D₃ and D₄, resistors R₂ and R₃ and comparators CM₁ and CM₂. In this embodiment, capacitor C₂ has a capacitance four times larger than the capacitance C of capacitor C₁ of VCO 30.

Under control of zero phase restart 40, multiplexer M₁ selectively gates either the output of divider 24 or the ENDAT signal to gate G₁. For the purposes of describing delay circuit 32, assume that both inputs of gate G₁ are initially low. The inverted output of gate G₁ will be high. When gate G₁ is high, the voltage of its output is equal to V_(CC). Therefore, transistor Q₅ will be on and node V_(C) will have a voltage equal to:

    V.sub.G =V.sub.CC -V.sub.BE.                               Eq. 41

Diode D₃, resistor R₂ and current source I₈ produce a voltage drop V_(R) over resistor R₂ equal to the voltage drop V_(R) over resistor R₁ in FIG. 6. Accordingly, a threshold voltage is formed at node V_(TH1) having a value of:

    V.sub.TH1 =V.sub.CC -V.sub.BE -V.sub.R.                    Eq. 42

Since

    V.sub.G >V.sub.TH1,                                        Eq. 43

the output of comparator CM₁ is low. The output of gate G₂ is low because of the high signal fed forward from the output of gate G₁. The output of gate G₂ is coupled to phase/frequency detector 34 of FIG. 1.

Diode D₄, resistor R₃ and current source I₉ produce a voltage drop over resistor R₃ equal to three-fourths of the voltage drop V_(R) over resistor R₁ in FIG. 6. Accordingly, a threshold voltage is formed at node V_(TH2) having a voltage of: ##EQU7## Since

    V.sub.G >V.sub.TH2,                                        Eq. 45

the output of comparator CM₂ is also low. The output of gate G₃ is low because of the high signal fed forward from the output of gate G₁. The output of gate G₃ is coupled to zero phase restart 40 of FIG. 1.

When the output of multiplexer M₁ goes high, the output of gate G₁ goes low. This low output is fed forward to gates G₂ and G₃, causing their respective outputs to go high. The high output of gate G₂ is fed back to gate G₁, which causes the outputs of gates G₂ and G₃ to be latched high until released by comparators CM₁ and CM₂, respectively.

When gate G₁ goes low, transistor Q₅ turns off. Current source I₇, which is modulated by transconductance amplifier T of FIG. 6 to produce current I, will begin to pull down the voltage at node V_(G) at a rate given by: ##EQU8## At some point, the voltage at node V_(G) will become less than the voltage at node V_(TH2). At this point, the output of CM₂ will go high and the output of gate G₃ will go low. At a later point, the voltage at node V_(G) will become less than the voltage at node V_(TH1). At this point, the output of CM₁ will go high and the output of gate G₂ will go low. At this point, if the output of multiplexer M₁ is high, transistor Q₅ will turn on. If the output of multiplexer M₁ is not high, the voltage at node V_(G) will continue to drop until the output of multiplexer M₁ goes high or the voltage of node V_(G) drops low enough to turn transistor Q₅ back on.

In producing the delay interval required for phase detector 34, the voltage at node V_(G) starts at:

    V.sub.G =V.sub.CC -V.sub.BE,                               Eq. 47

and drops to:

    V.sub.G =V.sub.TH1 =V.sub.CC -V.sub.BE -V.sub.R. Eq. 48

Accordingly, the voltage drop is V_(R). Since the rate at which the voltage at node V_(G) drops is: ##EQU9## the delay interval is: ##EQU10## This delay interval is equal to the oscillation period T_(V) of VCO 30 (derived at Equation 40) and will remain equal for any selected oscillation frequency. With very little additional circuitry, delay circuit 32 provides an accurate delay interval by recreating the oscillation period of VCO 30.

FIG. 9 is a timing diagram showing the voltage at node V_(G) and the outputs of gates G₂ and G₃ with respect to the output of multiplexer M₁. In FIG. 9, pulses 54, 56 and 58 from the output of multiplexer M₁ are of unequal duration.

Delay circuit 32 delays the leading edge of edge of the output of multiplexer M₁, with the delays reflected in the trailing edges of the outputs of gates G₂ and G₃. The rising edge of the output of gate G₂ notifies phase/frequency detector 34 of FIG. 1 that a phase comparison must be performed. The trailing edge of output gate G₂, which is the delayed edge, is the edge which is compared to an edge from divider 42 of FIG. 1.

With reference to FIGS. 8 and 9, the leading edge of pulse 54 causes the output of gate G₁ to go low, thereby turning off transistor Q₅ and latching the output of gates G₂ and G₃ high. The voltage at node V_(G) begins to drop. Before the voltage at node V_(G) drops to V_(TH2), the output of multiplexer M₁ goes low. At this point, the outputs of gates G₂ and G₃ are held high by the feedback loop between gates G₂ and G₁. The voltage at V_(G) continues to drop, with the output of G₃ going low when the voltage at node V_(G) equals the voltage at node V_(TH2) and the output of G₂ going low when the voltage at node V_(G) equals the voltage at node V_(TH1). When the output of gate G₂ goes low, the feedback loop between G₁ and G₂ is broken. Since the output of multiplexer M₁ is low, the output of gate G₁ is high and transistor Q₅ is turned back on.

Pulse 56 causes a similar response, except that pulse 56 does not go low until after the output of gate G₂ goes low. Therefore, the output of multiplexer M₁, not the feedback from gate G₂, turns transistor Q₅ on.

pulse 58 also causes a similar response at gates G₂ and G₃. However, pulse 58 remains on so long that the voltage at node V_(G) drops low enough to cause transistor Q₅ to turn back on.

The data separator of the present invention provides an improvement over data separators of the prior art. By utilizing a delay circuit to provide a delayed read signal to the phase/frequency detector, the zero phase restart circuit is able to anticipate the impending phase comparison and start the VCO such that the first transition in the signal derived from the VCO output signal coincides with the transition in the delayed read signal.

Another improvement over the prior art is realized by using a frequency synthesizer to control the delay circuit. Since the frequency synthesizer must be provided anyway to generate a write clock during write operations, very little additional circuitry is required to allow the frequency synthesizer to control the delay circuit during read operations. Not only does the frequency synthesizer accurately control the delay circuit, it also allows the delay interval of the delay circuit to be dynamically varied, which is required to support zone density recording.

By adding additional comparators coupled to different threshold voltages, any number of delay intervals can be derived from the delay circuit. Accordingly, the delay circuit can be utilized to provide any delay interval required of the read signal during a read operation.

The delay circuit of the present invention also provides a significant design advantage. During design of the delay circuit, the resistance of a resistor which determines a threshold voltage can be changed without affecting other delay intervals provided by the delay circuit. Accordingly, if a prototype of an integrated circuit having the delay circuit of the present invention proves to have an error in a delay interval, the threshold voltage which determines that delay interval can by easily and accurately changed by changing the resistance of the resistor which determines the threshold voltage. The other delay intervals provided by the delay circuit will be unaffected by the resistance change and therefore will not require additional attention from the designer.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A data separator which responds to a read request signal by extracting data embedded in a read signal that is derived from a transducer signal which is generated by a transducer moving proximate a storage media, the data separator comprising:controlled oscillator means for producing an oscillator output signal having an oscillation frequency which varies in response to a frequency control signal, the controlled oscillator means responsive to a stop signal which stops oscillation and a start signal which starts oscillation, wherein a start interval exists between assertion of the start signal and oscillation of the controlled oscillator means; means for deriving a read clock signal from the oscillator output signal; delay means for receiving the read signal and producing first and second delayed signals; restart means responsive to the read request signal and the first delayed signal, for asserting the stop signal based on the read request signal and thereafter asserting the start signal based on a transition from one signal level to another signal level in the first delayed signal; phase detection means for controlling a phase difference between the second delayed signal and the read clock signal by modulating the frequency control signal; and data extraction means for extracting data from the read signal by framing data in the read signal using the read clock signal.
 2. The data separator of claim 1 wherein the second delayed signal is delayed with respect to the first delayed signal by an interval approximately equal to the start interval to cause the oscillator output signal to start oscillating in a desired phase relationship with the second delayed signal upon assertion of the start signal.
 3. The data separator of claim 2 wherein the start interval is approximately one-fourth of an oscillation frequency period of the oscillator output signal.
 4. The data separator of claim 2 wherein the delay means delays the second delayed signal by approximately one-half of the read clock signal oscillation period.
 5. The data separator of claim 2 wherein the delay means delays the second delayed signal by approximately one read clock signal oscillation period.
 6. The data separator of claim wherein the means for deriving a read clock signal from the oscillator output signal comprises a frequency divider which receives the oscillator output signal and produces the read clock signal, wherein the read clock signal has a frequency equal to the oscillation frequency divided by an integer N.
 7. The data separator of claim 6 wherein N is one.
 8. The data separator of claim 6 wherein N is two.
 9. The data separator of claim 1 and further comprising:phase comparison anticipation means for signaling the phase detection means to perform a phase comparison based on a transition from one signal level to another second signal level in the read signal.
 10. The data separator of claim 1 wherein the delay means comprises:a frequency synthesizer which provides a write clock during write operations, the frequency synthesizer comprising:reference frequency means for providing a frequency reference signal having a reference frequency; second controlled oscillator means for producing a second oscillator output signal having a second oscillation frequency which varies with a magnitude of a second frequency control signal, the second controlled oscillator means also providing a delay control signal indicative of the magnitude of the second frequency control signal; means for deriving a first signal from the frequency reference signal; means for deriving a second signal from the second oscillator output signal; and second phase detection means for controlling a phase difference between the first and second signals by modulating the second frequency control signal; and variable delay means coupled to the delay control signal, for delaying the read signal to produce the first and second delayed signals which are delayed by intervals controlled by the delay control signal.
 11. The data separator of claim 10 wherein the reference frequency means comprises a crystal oscillator.
 12. The data separator of claim 10 wherein the variable delay means and the second controlled oscillation means reside on a common integrated circuit.
 13. The data separator of claim 10 wherein the means for deriving a first signal from the frequency reference signal comprises a second frequency divider which receives the frequency reference signal and produces the first signal, wherein the first signal has a frequency equal to the reference frequency divided by an integer A.
 14. The data separator of claim 13 wherein the means for deriving a second signal from the second oscillator output signal comprises a third frequency divider which receives the second oscillator output signal and produces the second signal, wherein the second signal has a frequency equal to the second oscillation frequency of the second oscillator output signal divided by an integer B.
 15. The data separator of claim 14 wherein the integers A and B are dynamically selectable to cause the second oscillation frequency of the second oscillator output signal to equal the reference frequency multiplied by the integer B and divided by the integer A.
 16. The data separator of claim 10 wherein the delay means further comprises:means for deriving a synthesizer clock from the second oscillator output of the second oscillator means; means for generating a select signal from the read request signal; and multiplexer means for receiving the select signal and selectively coupling either the synthesizer clock or the read signal to the variable delay means.
 17. The data separator of claim 10 wherein the second frequency control signal is a voltage signal.
 18. The data separator of claim 17 wherein the second controlled oscillator means comprises:transconductance amplification means, for receiving the voltage signal and producing a current I1; a capacitance C1; and a regenerative feedback path which alternately charges and discharges the capacitance C1, wherein the second oscillation frequency of the second oscillation means is based on a first charge rate of capacitance C1.
 19. The data separator of claim 18 wherein the variable delay means comprises:means for producing a current I2 based on the delay control signal, wherein the current I2 has a known relationship to the current I1; a capacitance C2 which is charged by the current I2 at a second charge rate; a first comparator which compares a voltage across the capacitance C2 with a first threshold voltage, wherein the first delayed signal has a delay interval equal to an interval required to charge the second capacitance from a starting voltage to the first threshold voltage at the second charge rate; and a second comparator which compares the voltage across the capacitance C2 with a second threshold voltage, wherein the second delayed signal has a delay interval equal to an interval required to charge the second capacitance from the starting voltage to the second threshold voltage at the second charge rate.
 20. A method for responding to a read request signal by starting a voltage controlled oscillator in a data separator in a desired phase relationship with a read signal which is derived from a transducer signal that is generated by a transducer moving proximate a storage media, the method comprising:receiving the read request signal; receiving the read signal; asserting an oscillator start signal based on the read request signal and based on a transition from one voltage level to another voltage level in the read signal, wherein assertion of the oscillator start signal causes the voltage controlled oscillator to start oscillating and wherein a start interval exists between assertion of the oscillator start signal and oscillation of the voltage controlled oscillator; delaying the read signal by approximately the oscillator start interval to produce a delayed read signal; determining a phase error by comparing a phase of a signal derived from the voltage controlled oscillator with a phase of the delayed read signal; and controlling the phase error by controlling the voltage controlled oscillator. 